Avoid negative thickness values

Signed-off-by: Santi Paprika <santi.gonzalez.cs@gmail.com>

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/Shadow/DirectionalLightShadow.azsli

@@ -154,7 +154,10 @@ float DirectionalLightShadow::GetThickness(uint lightIndex, float3 shadowCoords[
              shadowCoord.y >= 0. && shadowCoord.y * size < size - PixelMargin && shadowCoord.z < (1. - DepthMargin))
         {
             const float depthBufferValue = shadowmap.Sample(PassSrg::LinearSampler, float3(shadowCoord.xy, indexOfCascade)).r;
-            const float deltaDepth = abs(shadowCoord.z - depthBufferValue);
+            
+            // Normalized thickness (avoid negative values given by to precission or shrinking offsets)
+            const float deltaDepth = max(shadowCoord.z - depthBufferValue,0.0);
+
             const float viewSpaceThickness = ViewSrg::m_directionalLightShadows[lightIndex].m_far_minus_near * deltaDepth;            
             return viewSpaceThickness;
         }

Gems/Atom/Feature/Common/Assets/ShaderLib/Atom/Features/Shadow/ProjectedShadow.azsli

@@ -274,8 +274,9 @@ float ProjectedShadow::GetThickness()
             float3(atlasPosition.xy * invAtlasSize, atlasPosition.z),
             /*LOD=*/0
             ).r;
-            
-        const float viewSpaceThickness = abs(UnprojectDepth(m_shadowIndex, m_shadowPosition.z) - UnprojectDepth(m_shadowIndex, depthValue)); 
+        
+        // Denormalized thickness (avoid negative values given by to precission or shrinking offsets)
+        const float viewSpaceThickness = max(UnprojectDepth(m_shadowIndex, depthValue) - UnprojectDepth(m_shadowIndex, m_shadowPosition.z), 0.0); 
         return viewSpaceThickness;    
     }